This invention relates to a friction stepless speed change device for general purpose being suitable for use in industrial machines, automobiles, other vehicles and the like.
The friction stepless speed change devices concerning with the invention are classified into cone, disc, ring and spherical surface transmissions.
With any of the friction stepless speed change devices above described, the stepless speed change transmission is effected by changing rotating radii of friction transmission contact points in a stepless manner. The friction transmission contact points are divided into two kinds of external and internal contact types.
The external contact types are disclosed, for example, in Japanese Utility Model Application Publication No. 49-29,168 and Japanese Patent Application Publication No. 46-42,249. On the other hand, the internal contact types are disclosed, for example, in Japanese Utility Model Application Publication No. 46-34,919 and Japanese Patent Application Publication No. 41-5,765.
In the external contact type, the transmission is accomplished by contact between two convex surfaces, whose contact surfaces are wide, belt-like surfaces along contact orbits corresponding to pitch lines due to contact pressures.
As a result, positive and negative slips occur on outer and inner sides of the contact orbit so that such slips result in internal friction losses which lower the transmission efficiency.
Moreover, the case of which either the driving or driven side has a small curvature rotor, when the difference in rotating radii of the driving and driven sides becomes large, the above positive and negative slips rapidly increase. As a result, the transmission efficiency is lowered further.
Therefore, the friction stepless speed change device of the external contact type has a disadvantage in that the transmission efficiency is low due to the external contact.
On the other hand, with the device of the internal contact type, the transmission is effected by contact between a concave surface and a convex surface. Contact orbits are narrow line-shaped surfaces.
Consequently, the device of the internal contact type is superior to the external contact type device in that it has, less internal friction losses and high transmission efficiency.
With the device of the internal contact type, however, any input and output shafts surrounding the other shaft and one shaft for supporting friction transmission members must be movable.
Accordingly, owing to the construction, it is difficult to support rigidly the shaft performing translational movements.
With the device of the internal contact type, moreover, the friction discs are in point contact only at one point and high contact pressure therebetween is required, with the result that the friction discs are apt to open out of a parallel position.
In other words, even with the internal contact type, it is difficult to maintain the friction discs in parallel with each other so that contact pressure becomes unstable. As a result, transmission of torque is obstructed and hence transmission efficiency lowers. This disadvantage is acute in multiple-friction disc construction.